1. Field of the Invention
This invention relates to the preparation of lithium soap thickened greases, including mixed lithium-calcium soap thickened greases.
2. Description of the Related Art
Lithium soap thickened greases have been known for many years. Typically, the lithium soaps are derived from C.sub.10-24, preferably C.sub.15-18, saturated or unsaturated fatty acids or derivatives thereof. One particular derivative is hydrogenated castor oil, which is the glyceride of 12-hydroxystearic acid. 12-Hydroxystearic acid is a particularly preferred fatty acid.
Conventional processes for preparing lithium 12-hydroxystearate thickened greases have typically involved a step of heating an oil based reaction mass to above 200.degree. C. in order to melt the lithium 12-hydroxystearate. Temperatures above 200.degree. C. are not readily accessible with low pressure steam heating. Low pressure steam heating is a very convenient and widely used form of heating employed by grease makers.
U.S. Pat. No. 2,695,878 relates to lithium greases based on lithium soaps of 12-hydroxystearic acid. It describes the problem of conventional processes which require heating of reaction masses containing lithium soaps of 12-hydroxystearic acid, usually including most of the oil, to about 425.degree. F. (218.degree. C.), at which temperature the mass is fluid. Unless this high temperature is attained, the consistency of the grease is low so that excessive proportions of soap are required for a given penetration grease and the appearance and texture of the grease are poor. The solution to this problem propounded by U.S. Pat. No. 2,695,878 involves heating 12-hydroxystearic acid, either alone or in the presence of a trace of mineral acid. to about 150.degree. to 300.degree. F. (66.degree. to 149.degree. C), and holding at this temperature for a few hours to obtain polymerised 12-hydroxy- stearic acid. Lithium base, e.g. lithium hydroxide monohydrate, is mixed with the polymerised acid, preferably in the presence of oil, and the heated mixture is agitated until substantially all of the water has been removed (around 220.degree. to 250.degree. F.) (104.degree. to 121.degree. C). Heating is then continued in the presence of oil until the temperature of the soap-oil mixture has reached 300.degree. to 330.degree. F. (149.degree. to 166.degree. C.).
U.S. Pat. No. 2,697,693 relates to the manufacture of lithium soap greases and lists various disadvantages of the conventional process which is stated to involve heating a mixture of lithium 12-hydroxystearate and oil to above about 400.degree. F. (204.degree. C.), e.g. 425.degree. F. (218.degree. C.). These disadvantages all relate to the use of such high temperatures. U.S. Pat. No. 2,697,693 avoids these temperatures by subjecting a mixture comprising lubricant base (oil). lithium hydroxy soap constituents (e.g. lithium hydroxide hydrate, and 12-hydroxystearic acid or hydrogenated castor oil) and at least about 1 percent of water to a temperature of at least about 275.degree. F. (135.degree. C.), preferably from about 300.degree. F. (149.degree. C.) to about 330.degree. F. (166.degree. C.), and super-atmospheric atmospheric pressure, agitating the mixture, relieving the pressure after the mixture has reached that temperature by venting the steam therefrom, and maintaining the mixture at substantially that temperature while venting the steam. The resulting grease may advantageously be smoothed by milling, e.g. in a colloid mill.
U.S. Pat. No. 2,898,298 also mentions the disadvantage of the prior art processes requiring temperatures of about 400.degree. F. (204.degree. C.) to 425.degree. F. (218.degree. C.), and provides a lithium soap grease based on the lithium soap of a glyceride of a soap-forming hydroxy fatty acid such as the glyceride of 12-hydroxystearic acid, e.g. hydrogenated castor oil, optionally containing 10 to 20% of a lithium soap of a conventional grease-making fat, fatty acid or other soap-forming organic acid. The grease is made at a maximum temperature of about 330.degree. F. (166.degree. C.), e.g. by agitating a mixture of oil, fat and lithium hydroxide. oxide or carbonate at about 180.degree. to 220.degree. F. (82.degree. to 105.degree. C.) for 20 to 30 minutes to effect saponification,followed by dehydrating by raising the temperature to above 260.degree. F. (127.degree. C.), preferably to about 300.degree. to 330.degree. F. (149.degree. to 166.degree. C.). The resulting unhomogenised grease is then finished by milling in a colloid mill to provide gel strength in terms of ASTM penetration for the homogenised grease equivalent to about 25 to 125 penetration points increase in hardness relative to that of the unhomogenised grease.
U.S. Pat. No. 3,891,564 relates to the preparation of mixed lithium-calcium soap thickened greases, and provides a non-melt process wherein a mixture of lithium hydroxide or oxide e.g. lithium hydroxide monohydrate), water, oil base, saponifiable material (e.g. 12-hydroxystearic acid, stearic acid, hydrogenated castor oil, myristic acid, preferably 12-hydroxystearic acid) and calcium hydroxide or oxide (e.g. hydrated lime) are heated together up to but not exceeding the melting point of the calcium soap. The temperature must thus be kept below 293.degree. F. (145.degree. C.)), preferably at a temperature from 230.degree. to 293.degree. F. (100.degree. to 145.degree. C.).
U.S. Pat. No. 2,697,693 defines the "yield" of a grease making process as "the relationship between the amount of soap employed and the amount of oleaginous vehicle employed to produce a given quantity of grease of desired properties. Thus, the smaller the amount of soap employed to obtain a given quantity of grease of a particular consistency, the higher is said to be the "yield"." One appropriate test of consistency is the grease penetration test, ASTM D217.
There has now surprisingly been found a straightforward process for preparing lithium soap thickened greases at temperatures accessible by low pressure steam heating, which enables good yield to be achieved.